the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Nov 2025
Marine Cloud Brightening of Cumulus Clouds: From the Sprayer to the Cloud
Abstract. Marine Cloud Brightening (MCB) is a suggested solar radiation management approach to mitigate global warming by increasing the reflectance of clouds through the emission of additional aerosols. While stratocumulus are considered the preferred target for MCB, the present study investigates trade-wind cumulus clouds, which may be the dominant cloud type for certain regional MCB deployments. In this study, high-resolution large-eddy simulations with detailed Lagrangian cloud microphysics are used to assess the role of different aerosol sprayer heights on the efficacy of MCB. The study indicates that surface sprayers are the optimal placement, as they facilitate the most efficient dispersion of aerosol within the boundary layer, which increases the fraction of clouds affected by the sprayed aerosols, as well as the transport of the sprayed aerosols into the developing clouds, which increases the number of cloud droplets developing from the sprayed aerosols.
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- RC1: 'Comment on egusphere-2025-5575', Michael Diamond, 22 Dec 2025
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RC2: 'Comment on egusphere-2025-5575', Anonymous Referee #2, 15 Jan 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5575/egusphere-2025-5575-RC2-supplement.pdf
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AC1: 'Comment on egusphere-2025-5575', Johannes Kainz, 26 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5575/egusphere-2025-5575-AC1-supplement.pdf
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AC2: 'Comment on egusphere-2025-5575', Johannes Kainz, 26 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5575/egusphere-2025-5575-AC2-supplement.pdf
Status: closed
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RC1: 'Comment on egusphere-2025-5575', Michael Diamond, 22 Dec 2025
The authors use an LES coupled to a Lagrangian particle model to investigate the optimal sprayer placement for the dispersion and eventual activation of sea salt aerosol for marine cloud brightening in trade cumulus regions. They find that near-surface sprayers are most effective because they take advantage of the greater turbulence lower in the boundary layer and activate as cloud droplets at cloud base/are spread within cloudy updrafts efficiently. Cloud-layer spraying leads to lateral aerosol entrainment, which is inefficient for activation. Only the deepest clouds reach the aerosol plume and entrain the injected salt, leading to a non-physical relationship between cloud water and aerosol in the higher altitude cases. Even with this bias in the rCRE calculations, the surface sprayer is by far the most effective at brightening the scene. The analysis is well-done and presented appropriately. The writing is sharp and concise. I recommend publication following minor revisions addressing the comments below. -Michael Diamond
- Line 6: Near-surface?
- Line 4: Is the vertical domain really 20,480 m? Seems like it’s a bit over 3,500 m?
- Figure 2: What is the time of the snapshot?
- Line 92: Relative CRE?
- Figure 3: How is cloud fraction defined here?
- Figure 3: Why is rCRE not shown? You spend a lot of time talking about how to calculate it and show all its components, but it’s somehow missing itself…
- Figure 3: Can ensemble spread be indicated?
- Line 127: 3h instead of g?
- Line 180: It seems odd to talk about the GBR campaign being “proposed” by the 2024 review… Is there a more classic citation possible here? E.g., the 2019 “Reef Restoration and Adaptation Program: Environmental Modelling of Large Scale Solar Radiation Management. A Report provided to the Australian Government by the Reef Restoration and Adaptation Program” document. McDonald et al. (2019) or Condie et al. (2021) could also be appropriate.
McDonald, J., McGee, J., Brent, K., & Burns, W. (2019). Governing geoengineering research for the Great Barrier Reef. Climate Policy, 19(7), 801-811. https://doi.org/10.1080/14693062.2019.1592742
Condie, S. A., Anthony, K. R. N., Babcock, R. C., Baird, M. E., Beeden, R., Fletcher, C. S., Gorton, R., Harrison, D., Hobday, A. J., Plaganyi, E. E., & Westcott, D. A. (2021). Large-scale interventions may delay decline of the Great Barrier Reef. R Soc Open Sci, 8(4), 201296. https://doi.org/10.1098/rsos.201296 - Lines 205-210: Changing to talking about some example W m-2 values could be helpful here. It’s hard to get an intuitive feel for what is or isn’t an important difference in rCRE.
Citation: https://doi.org/10.5194/egusphere-2025-5575-RC1 -
RC2: 'Comment on egusphere-2025-5575', Anonymous Referee #2, 15 Jan 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5575/egusphere-2025-5575-RC2-supplement.pdf
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AC1: 'Comment on egusphere-2025-5575', Johannes Kainz, 26 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5575/egusphere-2025-5575-AC1-supplement.pdf
-
AC2: 'Comment on egusphere-2025-5575', Johannes Kainz, 26 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5575/egusphere-2025-5575-AC2-supplement.pdf
Data sets
Marine Cloud Brightening of Cumulus Clouds: From the Sprayer to the Cloud Johannes Kainz, Daniel Patrick Harrison, Fabian Hoffmann https://doi.org/10.5281/zenodo.17011767
Marine Cloud Brightening of Cumulus Clouds: From the Sprayer to the Cloud Johannes Kainz, Daniel Patrick Harrison, Fabian Hoffmann https://doi.org/10.5281/zenodo.17011767
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The authors use an LES coupled to a Lagrangian particle model to investigate the optimal sprayer placement for the dispersion and eventual activation of sea salt aerosol for marine cloud brightening in trade cumulus regions. They find that near-surface sprayers are most effective because they take advantage of the greater turbulence lower in the boundary layer and activate as cloud droplets at cloud base/are spread within cloudy updrafts efficiently. Cloud-layer spraying leads to lateral aerosol entrainment, which is inefficient for activation. Only the deepest clouds reach the aerosol plume and entrain the injected salt, leading to a non-physical relationship between cloud water and aerosol in the higher altitude cases. Even with this bias in the rCRE calculations, the surface sprayer is by far the most effective at brightening the scene. The analysis is well-done and presented appropriately. The writing is sharp and concise. I recommend publication following minor revisions addressing the comments below. -Michael Diamond
McDonald, J., McGee, J., Brent, K., & Burns, W. (2019). Governing geoengineering research for the Great Barrier Reef. Climate Policy, 19(7), 801-811. https://doi.org/10.1080/14693062.2019.1592742
Condie, S. A., Anthony, K. R. N., Babcock, R. C., Baird, M. E., Beeden, R., Fletcher, C. S., Gorton, R., Harrison, D., Hobday, A. J., Plaganyi, E. E., & Westcott, D. A. (2021). Large-scale interventions may delay decline of the Great Barrier Reef. R Soc Open Sci, 8(4), 201296. https://doi.org/10.1098/rsos.201296